CN1934458A - Dynamic shimset calibration for B0 offset - Google Patents

Dynamic shimset calibration for B0 offset Download PDF

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Publication number
CN1934458A
CN1934458A CNA2005800084171A CN200580008417A CN1934458A CN 1934458 A CN1934458 A CN 1934458A CN A2005800084171 A CNA2005800084171 A CN A2005800084171A CN 200580008417 A CN200580008417 A CN 200580008417A CN 1934458 A CN1934458 A CN 1934458A
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Prior art keywords
shim
magnetic field
main
coil
imaging
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CNA2005800084171A
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Chinese (zh)
Inventor
W·R·丹尼尔斯
D·L·福克萨尔
G·D·德米斯特
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Koninklijke Philips NV
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Koninklijke Philips Electronics NV
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Publication of CN1934458A publication Critical patent/CN1934458A/en
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
    • G01R33/387—Compensation of inhomogeneities
    • G01R33/3875—Compensation of inhomogeneities using correction coil assemblies, e.g. active shimming

Abstract

A magnetic resonance imaging method comprising: determining a magnitude shift of a main B0 magnetic field responsive to energizing one or more shim coils (60) at selected shim currents; energizing the one or more shim coils (60) at the selected shim currents; and performing a correction during the energizing to correct for the determined magnitude shift of the main B0 magnetic field. Wherein the determining a magnitude shift comprises: computing one or more Maxwell terms of the magnetic field produced by energizing the one or more coils (60) at selected shim currents; and determining the magnitude shift of the main B0 magnetic field based on the computed one or more Maxwell terms.

Description

Be used for B 0The dynamic shimming of skew is set calibration
Technical field
Relate to magnetic resonance arts below, and find its concrete application in magnetic resonance imaging, will be in conjunction with concrete with reference to being described.But, find that also it benefits from accurately known main B of amplitude at magnetic resonance spectrum and other 0Application in the technology in magnetic field.
Background technology
In magnetic resonance imaging, produce temporary transient constant main B 0Magnetic field, it is that the space is uniform at least in the visual field.For bigger main B 0Magnetic field intensity is 3 teslas or higher for example, and it is very difficult to seek out enough homogeneitys.Main B 0Heterogeneity in the magnetic field can produce various image artefacts.For example, in echo-planar imaging, the heterogeneity of main field can cause the pixel-shift in the reconstructed image.During design for the more compactedness that obtains reduction hardware cost, scanner, to the compromise heterogeneity in magnetic field that also can cause of more open access or the like of object or patient.
Main B 0The heterogeneity in magnetic field can be utilized effective shimming adjustment and be modified, and wherein Zhuan Yong shimming (shim) coil produces the heterogeneity that auxiliary or shimming magnetic field compensates magnetic field that main magnet produces.The normally superconduction of main magnet, and shim coil resistance coil normally.In one embodiment, each shim coil produces the magnetic field with space distribution, the magnetic field that produces perpendicular to other shim coil on its function.For example, each shim coil can produce the magnetic field that has corresponding to the space distribution of Legendre polynomial or spherical harmonic component.
In order to calibrate shim current, do not applying under the voltage condition to shim coil, field probe or miscellaneous equipment, or the special-purpose resonance that scanner is carried out is used to measure main B 0The spatial component that the space distribution in magnetic field, space distribution are broken down into quadrature is the spherical harmonics item for example.Utilize corresponding shim coil, the orthogonal terms in the non-shimming magnetic field that should be increased is replenished, and the orthogonal terms that should be reduced produces reverse shimming and partly offset by applying voltage for corresponding shim coil simultaneously.
Typically, shim current is calibrated once in a while, for example when magnetic resonance scanner is installed, after the overhaul or the like.During magnetic resonance imaging, use the shim current calibration value of having deposited and improve main B 0The homogeneity in magnetic field.
At higher main B 0In for example about 3 teslas or the higher magnetic field, the magnetic of the object to be imaged for example magnetic susceptibility will make main B with increasing gradually in the magnetic field 0The magnetic field distortion.It is relevant that these are out of shape imaging object normally, and depend on that also the position of imaging object and object will be by the interesting areas of imaging.Under these circumstances, carry out dynamic shimming adjustment and will become favourable, wherein each specific imaging object is adjusted the shim coil electric current, and may work as and during imaging, adjust electric current when being moved by imaging region.
In order to carry out the shimming adjustment of the distortion that imaging object is caused, the magnetic field mapping pulse train of utilizing the magnetic field sensor that is arranged in the magnet or magnetic resonance imagine scanner to carry out uses the imaging object of original position to measure main B 0Magnetic field.Main B 0The mapping space in magnetic field distributes and is broken down into quadrature component, and suitable corrective shim coil magnetic fields is determined and uses.
Shim coil is designed to adjust the main B that points to along the main field axis of selecting 0Magnetic field.In the magnet of typical horizontal bore, this axle typically launches along the hole axle, and is designated as the Z axle; But vertical magnets or other geometrical configuration also can be used.Therefore, in principle shim coil be designed to produce be parallel to main field axis magnetic-field component (the Z axle that for example is parallel to horizontal bore magnet) can partly increase or partly offset main B selectively 0Magnetic field.But shim coil also produces some components transverse to the main field axis Z axle of horizontal bore magnet (for example perpendicular to).
These transverse shim magnetic-field components cause the B by the shimming master 0The magnitude shift in magnetic field, and cause the skew of resonant frequency thus.Shimming induced field magnitude shift depends on the amplitude of the shim current that applies.This magnetic field amplitude skew is to depending on that the imaging technique with accurate home court is debatable.For example, at echo-planar imaging, the imaging of compact spiral k-space tracking, chemical shift are selected in excitation and some other technology, and the magnitude shift of the home court that is caused by shimming can produce pixel-shift or other harmful image artefacts.
The present invention considers a kind of improved apparatus and method, and it can overcome above-mentioned restriction and other restriction.
Summary of the invention
According to an aspect, provide a kind of MR imaging method.In response to the main B that gives one or more shim coil energy supplies with selected shim current 0The magnitude shift in magnetic field is determined, and these one or more shim coils are by with this selected shim current energy supply.In the process of energy supply, carry out calibration to calibrate determined main B 0The magnitude shift in magnetic field.
According on the other hand, a kind of MR imaging apparatus is disclosed.Provide a kind of generation main B 0The method in magnetic field.One or more shim coils make this main B 0It is even that magnetic field becomes.A kind of device is provided, is used for determining this main B 0Magnetic responsiveness is in the magnitude shift of giving one or more shim coil energy supplies with selected shim current.A kind of device is provided, is used for giving these one or more shim coil energy supplies with this selected shim current.A kind of device is provided, is used in the energy supply process, carrying out calibration to calibrate determined main B 0The magnitude shift in magnetic field.
According to further on the other hand, a kind of magnetic resonance imagine scanner is disclosed.Main magnet produces main B 0Magnetic field, one or more shim coils optionally make this main B with selected shim current 0Magnetic field becomes evenly, and processor is carried out one and handled, and this processing comprises determines main B 0Magnetic responsiveness is in the magnitude shift of shimming optionally.
An advantage is to be convenient to patient spy's the shimming of deciding.
Another advantage is to be convenient to the dynamic shimming in the imaging process.
Further another advantage is improved image quality, and this is because by the main B of shimming 0Coincideing between magnetic field amplitude and radio-frequency (RF) transceiver are tuning.
By reading following DETAILED DESCRIPTION OF THE PREFERRED, more other advantage and Li Chu will become apparent to one skilled in the art.
The present invention can various assemblies and the various arrangements of assembly realize, and can various process operations and the various arrangements of process operation realize.Accompanying drawing only is used to represent the purpose of preferred embodiment, and is not interpreted as limitation of the present invention.
Description of drawings
Accompanying drawing 1 diagram shows the specific and/or dynamic main B of a kind of realization patient 0The magnetic resonance imaging system of magnetic field shimming.
Accompanying drawing 2 diagrams have been drawn the shimming of enhancing to main B 0The typical effect that the magnetic resonance frequency in magnetic field distributes.
Accompanying drawing 3 diagrams show main B 0The vector calculation of the magnitude shift that magnetic field amplitude is caused by shimming.
Accompanying drawing 4 diagrams show by four imaging regions to volume of interest respectively carries out the dynamic shimming adjustment that the shimming adjustment realizes.
Embodiment
With reference to the accompanying drawings 1, magnetic resonance imagine scanner 10 comprises shell 12, is used for limiting a common cylindrical scan vestibule (bore) 14, wherein places dependent imaging object 16.Main field coil 20 be placed on shell 12 in, and produce the main B of the central shaft 22 be parallel to scanner bore 14 0Magnetic field.In accompanying drawing 1, main B 0The direction in magnetic field is parallel to the z axle with reference to Cartesian coordinate system x-y-z.Main field coil 20 typically is superconducting coil, is placed on the inboard of low temperature shielding 24, although also can use resistive main magnet.
Shell 12 is also taken in or supports magnetic field gradient 30, and it is used for optionally along transverse in the plane of central shaft 22 or produce the magnetic field gradient of the central shaft 22 that is parallel to vestibule 14 along other selected direction.RF body coil 32 is further taken in or supported to shell 12, and it is used for optionally encouraging and/or detecting magnetic resonance.The optional coil array 34 that is placed on vestibule 14 inboards comprises a plurality of coils, shown in example coils array 34 in be specially four coils, although can use the coil of other quantity.Coil array 34 can be used as the phased array of the receiver of parallel imaging, is used as sensitivity encoding (SENSE) coil of SENSE imaging or the like.In one embodiment, coil array 34 is the array of surface coils of placing near imaging object 16.Shell 12 typically comprises the decoration liner 36 that limits scanner bore 14.
Coil array 34 is used to receive the magnetic resonance by whole body coil 32 excitations, and perhaps magnetic resonance can be by single coil stimulating and reception, for example by whole body coil 32.Can recognize that if one of coil 32,34 is used to transmit and receive, another of coil 32,34 can selectivity be omitted so.
Main field coil 20 produces main B 0Magnetic field, magnetic resonance imaging controller 40 magnetic manipulation field gradient controllers 42 are optionally giving magnetic field gradient coils 30 energy supplies, and being coupled to radio-frequency coil 32 or being coupled to the radiofrequency launcher 44 of coil array 34 so that optionally give radio-frequency coil or coil array 32,34 energy supplies shown in the operation.By optionally magnetic manipulation field gradient coil 30 and radio-frequency coil 32 or coil array 34, magnetic resonance produced and in interested at least a portion zone of imaging object 16 by space encoding.By applying selected magnetic field gradient by gradient coil 30, selected k-space tracking is flute card track for example, and a plurality of radiation paths or helical trajectory are by horizontal (traversed).Alternatively, imaging data is as being gathered along the projection of selected magnetic field gradient directions.In the image-forming data acquisition process, shown in radio frequency receiver 46 collection of being coupled to coil array 34 or being coupled to whole body coil 32 be stored in magnetic resonance samples in the MR data storer 50.
Imaging data is reconstructed into graphical representation by reconstruction processor 52.Under k-spatial sampling data conditions, can be used based on the reconstruction algorithm of fourier transform.Other reconstruction algorithm is for example rebuild based on FBP and also can be used according to the form of the magnetic resonance imaging data of being gathered.For the SENSE imaging data, the imaging data that reconstruction processor 52 is gathered from each coil is rebuild folded image and folded image is combined with coil sensitivity parameters to produce the reconstructed image of expansion.
The reconstructed image that is produced by reconstruction processor 52 is stored in the video memory 54, and can show on user interface 56, is stored in the nonvolatile memory, transmits, watches, stores, handles or the like by LAN (Local Area Network) or the Internet.User interface 56 also can make radiologist, technician or other operator of magnetic resonance imagine scanner 10 can communicate by letter with magnetic resonance imaging controller 40 to select, to change and to carry out the magnetic resonance imaging sequence.
Main field coil 20 produces main B 0Magnetic field, 3 teslas or higher preferably approximately, this magnetic field is uniform in the imaging volume of vestibule 14 basically.But because the machinery or the electron drift of scanner 10 assemblies, some heterogeneities may exist or may develop in time.The imaging amount distortion that is caused by this heterogeneity may depend on the position of imagings in the vestibule 14.In addition, when dependent imaging object 16 was inserted in the vestibule 14, the magnetic of imaging object can make main B 0The magnetic field distortion.
In order to improve main B 0The homogeneity in magnetic field, one or more shim coils 60 of being taken in or supported by shell 12 provide effective main B 0The magnetic field shimming is adjusted.In one embodiment, each shim coil produces shimming magnetic field, and it has the space distribution in the magnetic field that produces perpendicular to other shim coil on the function.For example, each shim coil can produce the magnetic field that has corresponding to the space distribution of spherical harmonic component.By optionally giving each shim coil 60 energy supplies, main B with selected shim current 0The heterogeneity in magnetic field is lowered.
Ideally, each shim coil produces Distribution of Magnetic Field in vestibule 14, and it only comprises B ZComponent does not have horizontal B XOr B YComponent promptly is parallel to the main B of parallel sensing of z direction 0The magnetic field in magnetic field.Select B ZComponent is to improve or partly to offset the main B of main field coil 20 generations 0Magnetic field is to calibrate inherent heterogeneity, distortion that imaging object 16 causes or the like.Especially, shim currents processor 62 determines that for one or more shim coils 60 suitable shim current is to reduce main B 0The heterogeneity in magnetic field.Shim currents processor 62 is based on the known structure of shim coil 60 and the suitable shim current of the heteropical Information Selection in magnetic field that is calibrated based on needs.Main B 0The heterogeneity in magnetic field can be determined by variety of way, for example by using the magnetic field mapping resonance of carrying out by scanner 10 to gather the magnetic field mapping, by reading the optional magnetic field sensor (not shown) that is placed in the vestibule 14, calculate or the like by the priori (priori) of the expection field distortion of the importing generation of imaging object 16 by execution.The magnetic-field measurement sequence can be mixed with verification master B periodically with field pulses 0Magnetic field amplitude is for example after every.Shim currents processor 62 control shimming controllers 64 are given one or more shim coil 60 energy supplies with selected shim current.
Only comprise B although expect the Distribution of Magnetic Field that each shim coil produces in vestibule 14 ZComponent, but because magnetic flux must be followed closed loop, so shim coil 60 typically also produces for example B of at least some remaining transverse magnetic field components at least a portion of vestibule 14 XAnd/or B YComponent.The consequence of these transverse magnetic field components has reduced main B in the shimming adjustment exactly 0The spatial inhomogeneity distribution while in magnetic field, main B 0Average or the intermediate magnitude in magnetic field | B| changes, and strengthens with the shimming adjustment of strengthening usually.
Resonant frequency at the space set point is provided by following formula:
f res＝γ|B(x，y，z)| (1)，
Wherein | and B (x, y, z) | be that (x, y, magnetic field amplitude of z) locating and γ are for the gyromagnetic ratio that is energized nuclear magnetic resonance (gyrometric ratio) in the position.Amplitude | and B (x, y, z) | depend on the total magnetic field, not only depend on B ZComponent.Utilize the Cartesian coordinate system shown in the accompanying drawing 1:
|B(x，y，z)| 2＝[B x(x，y，z)] 2+[B y(x，y，z)] 2+[B z(x，y，z)] 2 (2).
As an example, 1The H proton nuclei has gyromagnetic ratio γ=42.58MHz/Tesla, exists like this | and B (x, y, z) | during=3.0 teslas, resonant frequency is approximately f Res=128MHz.Formula (1) has been pointed out the amplitude distribution of the frequency distribution of magnetic resonance intensity corresponding to magnetic field in the imaging volume like this.
With reference to the accompanying drawings 2, it has drawn the distribution as the magnetic resonance intensity of frequency function, and is viewed to main B 0The shimming of the amplitude in magnetic field is adjusted effect and is illustrated.In accompanying drawing 2, be expressed as I as the magnetic resonance intensity distribution of the not shimming adjustment of frequency function 0(f), it adjusts centre frequency f at shimming not 0Locate relative broad and concentrated.The magnetic resonance intensity distribution I that adjusts of shimming not 0(f) width has reflected in the vestibule 14 the main B that shimming is not adjusted 0The substantial spatial heterogeneity in magnetic field.Along with using selected shim current to apply the shimming adjustment to reduce the heterogeneity in magnetic field, it is narrower that magnetic resonance intensity distribution becomes, and reflection has improved spatially uniform.In accompanying drawing 2, by the shimming adjustment, uniform magnetic field, space provides narrow magnetic resonance intensity distribution basically, is expressed as I s(f).
Yet, except being narrowed down basically, the magnetic resonance intensity distribution I that is adjusted by shimming s(f) also be offset, and had centre frequency f towards higher frequency s＞f 0For regulating the application that shimming is adjusted relatively continually, the skew of this resonant frequency can be a problem and can be caused image artefacts.Carry out in imaging process in the application of dynamic shimming adjustment, this frequency shift (FS) takes place in imaging process.
With reference to the accompanying drawings 3, because shimming is adjusted main B 0The vector calculation of the magnitude shift of magnetic field amplitude is drawn.The desired magnetic field of being adjusted by shimming has component B in the z of amplitude direction ZIf (x, y z) locate, and the magnetic field that main field coil 20 produces is less than B at given position Z, the shimming adjustment preferably improves that magnetic field to value B so ZSimilarly, if the magnetic field that main field coil 20 produces greater than B Z, the shimming adjustment is preferably partly offset that magnetic field to meet value B so Z
Like this, the magnetic field of being adjusted by shimming has the uniform B in space basically that runs through vestibule 14 shown in the accompanying drawing 3 ZComponent.Yet the transverse magnetic field components additional, that do not expect that any shimming adjustment produces is for example all along the component B that illustrates of x direction orientation x(+I 1) or the component B that illustrates x(I 2) be not illustrated and be used for determining desired shim current.Like this, as shown in Figure 3, if shim current+I 1Be required to produce B ZBut this shim current+I 1Produce the additional cross stream component B that does not expect x(+I 1), so the position (total magnetic field of z) locating is for x, y | B| (+I 1)=(B z 2+ [B x(+I 1)] 2) 0.5, it is greater than desired amplitude B ZSimilarly, if shim current-I 2Be required to produce B ZBut this shim current-I 2Produce the additional cross stream component B that does not expect x(I 2), so the position (total magnetic field of z) locating is for x, y | B| (I 2)=(B z 2+ [B x(I 2)] 2) 0.5, it is also greater than desired amplitude B ZCertainly, can recognize that any cross stream component, let it be forward or negative sense or its transversal orientation all will tend to increase the amplitude in the magnetic field of being adjusted by shimming.These do not expect cross stream component to influence the typical case very little, this be since shim components typically less than main B 0Component, and the character of vector magnitude operation only depends on the spatial vertical component less than largest component tinily.But it is zero that the requirement of magnetic lines of flux formation closed loop typically makes the arbitrary place of cross stream component in vestibule 14 undesirablely.
These transverse magnetic fields, and they are known as maxwell terms here to the influence of magnetic field sum vector amplitude.In some documents, they also are known as the Maxwell field or the field of deriving.
Return with reference to the accompanying drawings 1, magnitude shift processor 70 is determined main B 0The be supposed to magnitude shift that produces in response to giving one or more shim coil 60 energy supplies of magnetic field with shim currents processor 62 selected shim current.Magnitude shift processor 70 was carried out this and is calculated before shim coil 60 is by actual energy supply, with priori (priori) prediction that magnitude shift is provided.This priori computation can be finished by visiting predetermined magnitude shift correction card 72, the magnitude shift to various shim current and combinations of shim currents that this table storage is measured in advance.For example, the frequency function that magnetic resonance intensity distribution can be used as various shim current and combinations of shim currents is recorded, to determine the deviation frequency f to various shim current and currents combination s, as shown in Figure 2.Based on formula (1), main B 0The magnitude shift Δ in magnetic field | B 0| can calculate by following formula:
Δ|B 0|＝|B 0| shimmed-|B 0| unshimmed＝(f shimmed-f unshimmed)/γ (3)，
Wherein γ is still to the gyromagnetic ratio of tested nuclear magnetic resonance.Though this experimental technique is flat-footed, it needs to measure a large amount of combinations of shim currents usually.In addition, if selected combinations of shim currents is not included in the correction card 72, the progressive numerical interpolation of so potential calculating typically is used.
In another method, main B 0The magnitude shift in magnetic field is estimated with maxwell terms.This method is confirmed because shim coil 60 intentions produce the magnetic field along z direction orientation, inequality B so z＞＞B x, B yTypically be held.Just, along the field component of z direction typically much larger than magnetic-field component transverse to the z direction.Under this condition, the magnitude shift Δ | B 0| can be expressed from the next:
Δ | B 0 | ≅ [ K s 1 ] [ I s ] + [ K s 2 ] [ I s 2 ] + [ K s 4 ] [ I s 4 ] + . . . + [ K s 2 n ] [ I s 2 n ] - - - ( 4 ) ,
[I wherein s] be the shim current vector that imposes on shimming 60.Vector [I s] null element therefore represent that corresponding shimming is not by energy supply and to the magnitude shift Δ | B 0| not contribution.Coefficient vector [ 1K s] be the coefficient of zero order vector that is calibrated coefficient of shimming 60, and the direct B that is produced by each shimming 60 is described 0.Coefficient vector [ 2K s] be the first rank maxwell terms coefficient vector that is calibrated coefficient of shim coil 60, it describes the first maxwell terms effect that is produced by each shim coil 60.Vector [I s 2] be the vector that comprises the shim current square value of the shim current that imposes on shimming 60.In addition, vector [I s 2] in null element therefore represent that corresponding shimming is not by energy supply and for the magnitude shift Δ | B 0| not contribution.Similarly, coefficient vector [ 4K s] ... [ 2nK s] represent the 2nd maxwell terms coefficient to the n rank, and vector [I s 4] ... [I s 2n] represent shim current values to be raised to the vector of specified power.
Maxwell coefficients vector [" K s] be stored in the maxwell coefficients vector memory 74.In one embodiment, these coefficients are applied magnetic field shifted by delta under one or several shim current level respectively by measuring at each shimming | B 0| and be calibrated.Maxwell coefficients vector [" K to that shim coil s] unit has except that corresponding to by [the I of the null element the unit of energy supply shimming by use s n] vector formula (4) that shim coil of optimization coefficient and be calibrated.The magnitude shift of the shim coil that this calibration hypothesis is operated separately when two or more shim coils 60 are operated together adds and ground merges, and this is to simplify supposition easily.
Advantageously, in case shim coil 60 has been calibrated maxwell coefficients [" K s], to the main B of any selected combinations of shim currents basically 0The magnitude shift in magnetic field just can be calculated priori value as the value of the input value derivation of equation (4) by using selected shim current, even this combination is different from the combination of using in those calibrations.It is continuous function about shim current that empirical function that formula (4) provides relation is compared with the discrete value in being typically stored in correction card 72, and the progressive numerical interpolation of therefore potential calculating is not used usually.
Replace maxwell coefficients [" K s] empirical calibration, these coefficients can obtain from the first principles computations based on the geometric configuration of shim coil 60.Such first principles computations is performed, and for example uses the finite element model of the loop geometries of various simulation shim current and makes coefficient and analog result is consistent.
Main B by magnitude shift processor 70 calculating 0The magnitude shift Δ in magnetic field | B 0| be used in giving selected one or more shim coil 60 energy supply processes, carry out and revise with the B that decides 0The correction of the determined magnitude shift in magnetic field.In one embodiment, the magnitude shift Δ that calculates by magnitude shift processor 70 | B 0| give 82 energy supplies of D.C. shim coil and compensated by operation D.C. shimming controller 80.D.C. shim coil 82 is zero order shim coil, and it produces uniform magnetic field, space in vestibule 14 by energy supply the time.D.C shimming controller 80 is given 82 energy supplies of D.C. shimming and is offset the magnitude shift Δ basically with selected reverse shim current | B 0| (being assumed to be the true amplitude skew).D.C. shimming 82 is offset the true amplitude shifted by delta | B 0| to keep main B 0Magnetic field is a constant, even when the one or more shimming 60 of operation.
In another embodiment, magnitude shift processor 70 outputs are equivalent to main B 0The magnitude shift Δ in magnetic field | B 0| magnetic resonance frequency shifted by delta f ResAs shown in Equation (1), magnetic resonance frequency shifted by delta f ResEqual the magnitude shift Δ | B 0|, except conversion gyromagnetic ratio factor γ.Magnetic resonance frequency shifted by delta f by magnitude shift processor 70 outputs ResBe used as control signal (representing) and control the radio-frequency (RF) transceiver 44,46 that comprises radio-frequency (RF) transmitter 44 and radio frequency receiver 46 to guarantee that they are with corresponding to comprising the magnitude shift Δ by the virtual connections arrow in the accompanying drawing 1 | B 0| main B 0The magnetic resonance frequency operation in magnetic field.In other words, with reference to the accompanying drawings 2, the centre frequency of transmitter 44 is adjusted to by the frequency f of shimming sCan in receiver 46, similarly adjust.
Above-described arbitrary magnitude shift correction embodiment or their equivalent can be used with the convenient shimming of adjusting on frequent relatively basis.For example, can carry out the shimming adjustment, to solve each patient's different magnetic susceptibility characteristics to each patient.In addition, arbitrary described magnitude shift correction embodiment or their equivalent are convenient to carry out dynamic shimming adjustment in imaging process, wherein during the imaging of single patient the part, carry out the shimming adjustment on each lamella or other basis.
With reference to the accompanying drawings 4, imaging volume V surrounds the head and the trunk of imaging object 16.Imaging object 16 makes the not main B of shimming adjustment 0Magnetic field is out of shape in the spatial inhomogeneity distribution mode on whole imaging volume V.In accompanying drawing 4, this distortion is by drawing the not average main B of shimming adjustment 0Magnetic field | B (z) | represented that schematically it is average on each axial slice, as the function of axial slice position on the z direction.Although drawn the variation on the z direction, can recognize main B 0Magnetic field laterally also can be deformed on x and the y direction equally.Whole imaging volume V can be used as a unit and is adjusted by shimming, but the inhomogeneity effect of presentation space can be very difficult on large volume V.
In the dynamic shimming method of adjustment shown in the accompanying drawing 4, imaging volume V is divided into four region R along the z direction 1, R 2, R 3, R 4Some zones demonstrate more changes of magnetic field than other zone.In described example, region R 3, R 4Compare region R 1, R 2Has more changes of magnetic field.Each region R 1, R 2, R 3, R 4By shimming adjustment respectively.Just, to each zone, one or more shim current are selected to reduce main B in that zone basically 0The heterogeneity in magnetic field.Because the shimming adjustment concentrates on littler zone, therefore can carry out more accurate shimming adjustment to each zone.Work as region R 1During by imaging, be used for the selected shim current that shimming adjusts that zone to be used.Work as region R 2During by imaging, be used for the selected shim current that shimming adjusts that zone to be used.Work as region R 3During by imaging, be used for the selected shim current that shimming adjusts that zone to be used.Work as region R 4During by imaging, be used for the selected shim current that shimming adjusts that zone to be used.
Accompanying drawing 4 has also diagrammatically been drawn the average main B that adjusts at the shimming that averages on each axial slice in that lamella imaging process 0Magnetic field | B (z) |.The average main B that shimming is adjusted 0Magnetic field is uniformly basically in each zone, but shows the magnitude shift Δ | B 0|, it is not corrected in accompanying drawing 4.The average main B that shimming is adjusted 0Magnetic field | B (z) | curve do not comprise the compensation of optionally being undertaken by D.C. shim coil 82.Because each region R 1, R 2, R 3, R 4Usually adopt different selected shim current imagings, therefore each the regional magnitude shift Δ in four zones | B 0| vary in size.In described example, before shimming is adjusted in region R 3, R 4Imaging process in adopt bigger shim current to compensate magnetic field heterogeneity relatively large in those zones; Otherwise, showing the less heteropical region R in field 1, R 2Imaging process in adopt littler shim current.Therefore, region R 3, R 4The main B that middle shimming is adjusted 0Magnetic field and region R 1, R 2Compare and have bigger magnitude shift Δ | B 0|.Be suitable for each respective regions R by using magnitude shift processor 70 to calculate 1, R 2, R 3, R 4The magnitude shift Δ of each selected combinations of shim currents that the shimming adjustment is adopted | B 0|, adjust the magnitude shift Δ that changes in the imaging process at dynamic shimming | B 0| compensated.
Although accompanying drawing 4 shows four zones, each comprises a plurality of lamellas, can recognize that dynamic shimming adjustment technology can be applied to other sub-volumes.For example, dynamically the shimming adjustment can be employed on each slice basis, wherein before to that lamella imaging each axial slice is selected shim current.
Among the described before this embodiment, shim current is selected to reduce main B in the imaging region 0Magnetic field | heterogeneity.In case shim current is selected, by the magnitude shift Δ of those selected shim current generations | B 0| calculated, and to that magnitude shift Δ that calculates | B 0| correction be performed.Select the process of shim current, calculating magnitude shift and correction to be carried out respectively.
But, in the embodiment of other expection, select shim current, calculate Δ | B 0| and the process of proofreading and correct partly or wholly is integrated together.For example, comprise field uniformity component and magnitude shift component Δ by optimization | B 0| quality factor (figure of merit) determine shim current.In this embodiment, optimized simultaneously, carried out the selection and the magnitude shift Δ of shim current thus simultaneously by the shim current that minimizes or maximize quality factor, comprise the shim current that is used for shim coil 60 and D.C. shim coil 82 | B 0| correction calculation.
The shimming adjustment influences volume, and volume is carried out the measurement of resonant frequency, and these volumes typically demonstrate spatial coherence.Be offset diameter that measured conduct for example is positioned at magnet center to predetermined in one embodiment and be 20 centimetres reference spheroid average.Other contemplated embodiments can comprise for example (i) scope of planning imaging region subsequently of volume defining, (ii) be designated as the some parts of the central area of picture volume, (iii) object is by the health scope of imaging, perhaps be limited in the bigger designated volume, the (iv) typical volume that limits according to interested human dissection, or (the v) clear and definite zone that limits by the operator who carries out the MRI process.Many other qualifications are possible.
In addition, determine that by the resonant frequency shift that is caused by the energy supply shimming some modes are in conjunction with the selection of volume arbitrarily.Magnetic field skew or deviation ratio to one or more predetermineds can be defined.Skew can be endowed the feature of spatial coherence, for example by polynomial fitting or other spatial function.For example, this polynomial expression can be spheric harmonic function (sphericalharmonics), perhaps the space distribution that they can each shim coil of match maxwell terms separately.Determine skew or deviation ratio on each of some points that can be in the Discrete Mapping table, and represent storage as volume.
For illustrative purposes, further describe the specific embodiment of computation of maxwell terms now.Utilize the spheric harmonics expansion in magnetic field, main magnet coil 20 can mainly be used to produce B zOrder spherical harmonic, magnetic field gradient coils 30 can be utilized to produce or proofread and correct B zThe first rank spheric harmonic function item, magnetic shim coil 60 can be utilized to produce or proofread and correct B zThe second rank spheric harmonic function item.These second rank shimmings can refer to for example (x 2-y 2), xy, xz, yz and z 2Its title of spatial coherence coupling of each these shimming is except being called as z 2Shimming outside, its generation has B z=(z 2-0.5* (x 2+ y 2)) the field of spatial dependence function.These second rank shimmings for each, corresponding transverse field can be determined.Transverse field B for each shimming xAnd B yBe constrained to gang and separate, this is because Maxwell's formula must be satisfied in magnetic field.At B xAnd B yFunction may select to exist some freedom.
In shim coil mechanically is based upon embodiment on the cylindrical surface, during some symmetry can be incorporated into and separate.By these symmetry constraint being applied to candidate's transverse field B xAnd B yFunction, the spatial coherence of each function is determined.For (x 2-y 2) shimming, separating under these symmetry constraint is B x=2xz and B y=-2yz.For the xy shimming, separating under these symmetry constraint is B x=2yz and B y=2xz.For z 2Shimming, separating under these symmetry constraint is B x=-xz and B y=-yz.Can carry out similarly other second rank shimming and determine.
In one embodiment, by utilizing formula (2) and, calculating total amplitude B skew to the shim current in arbitrary given shim coil to volume integral.The power series expansion of square root function produces the coefficient of the power of shim current then.Have only even power can produce nonzero coefficient.
In another embodiment, to the vector field B=(B of each shimming x, B y, B z) and B zComponent is desired sets ratiometric conversion for.The vector of all conversion shimmings is added.The amplitude of vector sum is confirmed as the function of position x, y and z.Composite function to interested volume integral to provide a final skew B amplitude.
For the continuous more expansion at preceding embodiment of high-order shimming is flat-footed, and based on the description of front about the second rank shimming, can easily be carried out by those skilled in the art.
Can recognize that it can be useful and no matter cause the mechanism of frequency shift at first being used for the correction of resonant frequency of deviation that shimming is caused.Therefore, viewed frequency shift (FS) may be to be caused by other mechanism, except above-described maxwell terms.The mechanical deflection that is caused by thermal effect relevant with electric current in electrostatic force or the resistive shim coils is other examples of mechanisms that may cause frequency change.These and other mechanism may demonstrate identical basic functional dependence, even they may be different from the effect of predetermined maxwell terms to a great extent on intensity.For example, may in other magnetic structure, cause small geometrical deviation for the shim coil energy supply of specific ball hamonic function, then cause basically square proportional frequency shift (FS) with shim current.Utilize such alignment apparatus and method and their direct modification can easily carry out the heteropical correction in the magnetic field that this empiric observation is arrived, aforesaid like this calibrating installation and method can easily be suitable for proofreading and correct the magnetic field heterogeneity that the empiric observation observing or measure arrives under the situation of uncertain main cause.
The present invention has been described in conjunction with the preferred embodiments.Apparently, can produce modification and variation reading and understand in the detailed description in front.Can recognize, the present invention includes all this modification and variations, as long as they drop in the scope of accessory claim book or its equivalent.

Claims (20)

1. MR imaging method comprises:
Determine main B 0Magnetic responsiveness is in the magnitude shift of giving one or more shim coils (60) energy supply with selected shim current;
Give this one or more shim coils (60) energy supply with this selected shim current; With
In the energy supply process, carry out to proofread and correct with to this main B that determines 0The magnitude shift in magnetic field is proofreaied and correct.
2. MR imaging method as claimed in claim 1, wherein the execution of Jiao Zhenging comprises:
The centre frequency of adjusting radio frequency receiver and transmitter component (44,46) is to corresponding to the B that comprises that this determines magnitude shift 0The magnetic resonance frequency in magnetic field.
3. MR imaging method as claimed in claim 1, wherein the execution of Jiao Zhenging comprises:
Give D.C. shim coil (82) energy supply with the D.C. shim current, be used for offsetting effectively the main B that this is determined 0The magnitude shift in magnetic field.
4. MR imaging method as claimed in claim 1, wherein the execution of Jiao Zhenging comprises:
Give one or more gradient coils (30) energy supply with to this main B that determines 0One or more first rank spheric harmonic function items of the magnitude shift in magnetic field are proofreaied and correct.
5. MR imaging method as claimed in claim 1, determine that wherein magnitude shift comprises:
One or more maxwell terms in the magnetic field that calculating produces by giving this one or more shim coils (60) energy supply with selected shim current; With
Based on one or more maxwell terms of this calculating, determine main B 0The magnitude shift in magnetic field.
6. MR imaging method as claimed in claim 1, determine that wherein magnitude shift comprises:
These one or more shim coils (60) for each are determined the one or more maxwell terms coefficients by the magnetic field that produces to that shim coil energy supply with corresponding selected shim current;
These one or more shim coils (60) for each multiply by the electric current that is increased to corresponding average power by (i) with each maxwell terms coefficient of that coil and obtain corresponding to one or more maxwell terms of this one or more maxwell terms coefficients of that coil and (ii) this maxwell terms addition is determined the magnitude shift contribution of that coil.
7. MR imaging method as claimed in claim 6, wherein these one or more shim coils (60) comprise a plurality of shim coils, and definite magnitude shift further comprises:
Add with ground and merge the magnitude shift contribution of these a plurality of coils (60) to determine main B 0The magnitude shift in magnetic field.
8. MR imaging method as claimed in claim 6, it is one of following to determine that wherein one or more maxwell terms coefficients comprise:
Based on the geometric configuration of coil, calculate the maxwell terms coefficient and
To fit to an expression formula with the magnetic field that reference current produces for the shim coil energy supply, this expression formula comprise by the parameterized one or more maxwell terms of corresponding one or more maxwell terms coefficients and, described maxwell terms coefficient is stored and is called in the process of the magnitude shift contribution of determining that coil subsequently.
9. MR imaging method as claimed in claim 1, wherein these one or more shim coils (60) comprise a plurality of shim coils, and definite magnitude shift comprises:
For each coil, determine the funtcional relationship between the skew contribution of shim current and that coil;
Selected shim current is input in this funtcional relationship to determine the skew contribution corresponding to selected shim current; With
Merging is contributed to determine magnitude shift corresponding to the skew of the selected shim current of a plurality of coils.
10. MR imaging method as claimed in claim 9 wherein merges the skew contribution and comprises:
Determine the skew contribution of each shim coil with vector form;
Add the skew composition contribution that merges each shim coil with ground; With
Determine corresponding to the selected shim current of these a plurality of coils (60) the amplitude of vector offset contribution.
11. MR imaging method as claimed in claim 1 further comprises:
The quality factor that comprise the shim current of the shim current of shim coil (60) and D.C. shim coil (82) by optimization are selected selected shim current, wherein carry out to proofread and correct to comprise to give D.C. shim coil (82) energy supply by the shim current of optimizing the optimization that quality factor obtain.
12. MR imaging method as claimed in claim 1 further comprises:
The Dynamic Selection shim current with during magnetic resonance imaging dynamically shimming adjust main B 0Magnetic field repeats to determine that to the selection each time of shim current magnitude shift, energy supply and execution proofread and correct.
13. MR imaging method as claimed in claim 1 further comprises:
Carry out the multi-slice MR imaging of imaging object; With
For each lamella, the shim current of selecting these one or more shim coils (60) with to that lamella dynamically shimming adjust main B 0Magnetic field for to that lamella imaging, is carried out and is determined magnitude shift, energy supply and execution correction.
14. MR imaging method as claimed in claim 1 further comprises:
To be divided into a plurality of imaging regions by imaging region;
For each imaging region, determine that selected shim current adjusts main B in that imaging region with shimming effectively 0Magnetic field, to each imaging region to selected shim current carry out respectively in response to the magnitude shift of giving one or more shim coils (60) energy supply with selected shim current determine adjust main B in that imaging region with shimming effectively 0Magnetic field; With
Gather the imaging data of each imaging region, wherein:
(i) the energy supply part that is used as imaging carry out and use selected shim current effectively the shimming adjustment by the main B in the imaging region of imaging 0Magnetic field and
(ii) the execution of Jiao Zhenging is with respect to being carried out by the determined magnitude shift of imaging region for that.
15. a MR imaging apparatus comprises:
Be used for producing main B 0The device in magnetic field (20);
Being used for shimming adjusts main B 0One or more shim coils (60) in magnetic field;
Be used for determining main B 0Magnetic responsiveness is in the device (70) of giving the magnitude shift of one or more shim coils (60) energy supply with selected shim current;
Be used for giving the device (64) of this and a plurality of shim coils (60) energy supply with selected shim current; With
Be used for during energy supply carrying out and proofread and correct with to determined main B 0The device that the magnitude shift in magnetic field is proofreaied and correct (44,80,82).
16. MR imaging apparatus as claimed in claim 15 is used for wherein determining that the device (70) of magnitude shift comprises a processor, the processing of its execution comprises:
Determine one or more maxwell terms coefficients of each shim coil;
Based on the main B of shim coil function calculation by each coil generation 0The magnitude shift in magnetic field, the function parameter that described function has comprise the selected shim current of one or more maxwell terms coefficients He that coil of that coil; With
Merge the main B that each coil produces 0The magnitude shift in magnetic field.
17. MR imaging apparatus as claimed in claim 15 is wherein proofreaied and correct actuating unit (44,80,82) and is comprised one of at least following:
Be used for exciting zero order shim coil (82) to adjust main B 0The device of the amplitude in magnetic field (80); With
Be used for being offset the device (44) of resonance excitation frequency.
18. a magnetic resonance imagine scanner comprises:
Be used for producing main B 0The main magnet (20) in magnetic field;
With selected shim current optionally shimming adjust main B 0One or more shim coils (60) in magnetic field; With
Execution comprises determines main B 0Magnetic responsiveness is in the processor (70) of the processing of the magnitude shift of selected shimming.
19. magnetic resonance imagine scanner as claimed in claim 18 further comprises:
Zero order shim coil (82), optionally by energy supply to offset determined main B 0Magnetic responsiveness is in the magnitude shift of selected shimming.
20. magnetic resonance imagine scanner as claimed in claim 18 further comprises:
Tunable radio frequency transceiver (44,46) is used for producing radiofrequency signal and detects the magnetic resonance signal that produces in response to the radiofrequency signal that is produced;
Wherein the processing of being carried out by processor (70) comprises that further calculating is corresponding to the main B that comprises determined magnitude shift 0The magnetic resonance frequency in magnetic field, tunable radio frequency transceiver (44,46) is transferred to the magnetic resonance frequency of calculating.
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